1. Field of the Invention
The present invention relates to a projection lens for a projector to enlarge and project a generated image on a screen or the like, and more particularly, to a projection lens for a pico-projector, to be installed in a small mobile device such as a mobile phone and to reduce a height of a lens array.
2. Description of the Related Art
As the electronic technology has been developed, the use of personal mobile devices, such as a mobile phone and a PDA, among others, has been rapidly increasing and the mobile devices with diverse functions of a MP3, a PMP, a camera and the like have been released.
In accordance with the multi-functional trends, a mobile device with the function of a micro projector known as a so-called pico-projector or nano-projector is launched.
In designing a lens for a small projector, it is important to achieve the lens in a minimum size and with a high-performance function. Further, the lens is to be low in cost and light in weight, to reduce distortion and magnification chromatic aberration, and to satisfy a relatively long back focus length.
For example, FIG. 1 illustrates a conventional lens unit 1 for a mini projector in which the full length is about 60 mm.
As illustrated in FIG. 1, the lens unit 1 for the mini projector comprises: a lens array 10 consisting of seven lenses, a prism 20 for color composition, a cover glass 30 and an image panel 40. The length from a 1st lens 11 to a 7th lens 17 of the lens array 10 is called the full length and the length from the 7th lens 17 (the final lens as arrayed) to the image panel 40 is called the back focus length (BFL).
In the conventional lens unit 1 for a mini projector as illustrated in FIG. 1, the full length is generally about 60 mm and the back focus length is about 32 mm. However, the back focus length may vary according to the design of the lens array 10.
To use the lens unit 1 for a mini projector according to the conventional is art shown in FIG. 1, in which the full length is about 60 mm, as a lens unit for a pico-projector in which the full length is about 10 mm, it is possible to consider to relatively reduce the lens unit 1 for a min projector at a rate of about 1:10.
However, in the case of reducing the lens unit 1 for a mini projector in which the full length is about 60 mm to the lens unit for a pico-projector in which the full length is about 10 mm, the lens array 10 may be changed to be reduced in design but there are many limitations in reducing the diameter of the lens, i.e., the height of the lens.